He got 10000 receiving yards already?
He got a 1,000 receptions already?
If not then
Being the greatest doesn't necessarily mean you have to have the longest career or be the biggest stat compiler. Not TE has ever been as dominant and had the impact Gronk has had during his short time in the NFL. He still has many years left in his career.
I'll do some assuming here, but I'm going to give him the benefit of the doubt and say he stays relatively healthy for the next four seasons. If he does that, he will very likely be SIXTH all-time in receiving TD's before he turns 31. Let's say he misses a season worth of games over that time period, he will likely reach that same milestone by the time he is 32 years old.
There is no upper echelon every down TE that blocks as well as him. His impact is insane.
I'd bet good money that he finishes in the top four ALL-TIME in receiving TD's, and I think if he can stay relatively healthy that he COULD possibly end up second. That's going to require him staying healthy, but I think it's definitely possible for Gronk.